BRIEF DESCRIPTION OF THE PRIOR ART
Tertiaryalkyl ethers are compounds desirably used in the blending of high octane premium unleaded gasolines. It is known to produce such ether compounds in an etherification reactor in which a lower, water soluble alkanol, such as methanol, is reacted with an isoolefinic compound derived from an isoparaffin to yield the tertiaryalkyl ether. The isoolefin compound is produced by charging an isoparaffin of the desired number of carbon atoms to a dehydrogenation zone in which the isoparaffin is dehydrogenated to the corresponding iso-olefin over a suitable dehydrogenation catalyst. A process by which ethers are prepared in the manner described is disclosed in U.S. Pat. No. 4,118,425. In the described etherification process, it is useful to recycle to the dehydrogenation reactor unreacted isoparaffins which have not been dehydrogenated in the dehydrogenation zone, and therefore not reacted in the etherification zone.
The advantage achieved by recycling isoparaffin compounds to the dehydrogenation reactor is impaired by the inclusion in the effluent from the etherification reactor of linear olefinic materials. These olefinic materials, such as, for example, butene-1, butene-2 and butadiene, pass through the etherification reactor intact after having been formed in small quantities in the dehydrogenation of the isoparaffin compounds to the corresponding isoolefins. Since the linear olefin compounds pass through the entire closed system without undergoing significant reaction, except for some conversion of the monounsaturated butenes to butadiene, they tend to build up as the cyclic process is continued. This build up will ultimately result in the lay down of coke on the dehydrogenation catalyst bed, thereby shortening the effective life of the catalyst prior to the time when it would otherwise require regeneration.
Some linear paraffins will usually be present in the isoparaffin charge to the dehydrogenation catalyst, and, while being partially converted to the normal olefin compounds described, will also, in part, continue through the dehydrogenation and etherification reactions in small quantities of the unchanged normal paraffins. Such compounds are also unaffected by the etherification reaction, and it is desirable to remove them from the recycle stream which is passed from the etherification reaction zone to the dehydrogenation reactor.
In U.S. Pat. No. 4,581,474, a stream which comprises a mixture of butene-1, butene-2 and isobutene is charged to a conventional methyltertiarybutyl ether (MTBE) plant, along with methanol. In the etherification reactor, the isobutene reacts with the methanol to yield MTBE. After recovery of the MTBE, the effluent stream, now enriched in linear butenes, is passed to an adsorption zone where a large portion of the butene-2 content of the stream is adsorbed. In this adsorption zone, a molecular sieve is utilized to relatively adsorb butene-2 from butene-1. The effluent from the adsorption unit is a butene-1 rich stream, and this is divided into two streams. One of these is subjected to a double bond isomerization to convert the butene-1 to butene-2. The butene-1 in the other of the two streams is isomerized to isobutene which is then recycled to the MTBE reactor to supplement the fresh isobutene charge stock. This patent is not concerned with the preparation of the initial or original isobutene charge stock to the MTBE reactor, and thus does not disclose the preparation of this material by dehydrogenation of the corresponding isobutane. Neither is this patent concerned, therefore, with the recycling of an isoparaffin recycle stream to a dehydrogenation reactor, or the removal of undesirable linear olefin compounds therefrom.